At present, electron beam tubes for treating and processing surfaces typically use 15 micron thick titanium foil as the electron beam window. This requires a minimum operating voltage of 150 kilovolts in order to just penetrate the foil thickness. Typical voltages of at least 175 kilovolts are thus used to allow for the additional energy to process the substrate material.
The consequences of running at these voltage levels (175 Kev) to cure thin coatings, i.e. less than 10 microns, is inefficient and a great energy waste due to the fact that most of the electron energy passes right through the coating to be cured. Such equipment is large and costly primarily due to the x-ray shielding requirements produced by the high energy electrons.
An electron beam tube for surface treatment which avoids high voltages is a remote ion source type. Such a tube generates a stripe-like electron beam which impinges on a surface in a stripe pattern. The beam may be used for surface treatment of materials, such as curing of adhesives, and radiation chemistry in general. An example of this type of beam tube is found in U.S. Pat. No. 4,910,435 to G. Wakalopulos, assigned to the assignee of the present invention. Also disclosed is a long electron beam source, shown in FIG. 5 of the '435 patent, wherein a plurality of offset ion sources produce a plasma cloud at the center of a long chamber. The long plasma cloud generates a long electron beam. The present invention is concerned with this type of electron beam tube wherein a stripe-like beam is generated.
In U.S. Pat. No. 4,468,282 A. P. Neukermans discloses a device resembling a cathode ray tube, but having a face with a narrow window made of a thin silicon carbide or silicon nitride film layer. The pencil-like beam emitted through the window is used in a printing application. Neukermans showed that tough thin film windows can be advantageously used in scanning beam tubes.
An object of the invention was to provide surface treatment with wide electron beams, especially for polymer crosslinking applications, with minimum energy loss at the tube window.